Suppressed formation of polycyclic aromatic hydrocarbons (PAHs) during pyrolytic production of Fe-enriched composite biochar

The pyrolytic production of Fe-enriched composite biochar is receiving increasing attention. However, understanding of the environmental risk from the polycyclic aromatic hydrocarbons (PAHs) potentially generated during composite biochar production is lacking. This study investigated the formation of PAHs from the pyrolysis of barley straw impregnated with FeCl₃ or Fe(NO₃)₃ at 350 °C, 500 °C, and 650 °C. The total amount of PAHs formation increased with increasing heating temperature. Most of the PAHs were concentrated in bio-oil (72.7-94.6%), with only a small fraction retained in biochar (1.7-11.1%) and in biogas (2.2-16.2%). Preloading FeCl₃ or Fe(NO₃)₃ onto the biomass greatly reduced PAH formation by up to 33% and 21%, respectively, compared to that obtained with biomass alone. The suppressed formation of PAHs was due to the generation of more reductive forms of Fe, such as Fe⁰ and FeO, in the O₂-starved pyrolysis atmosphere, which reduced C₂H₂ and C₆H₅OH, two important PAH precursors in hydrogen abstraction acetylene addition reactions. Although Fe loading reduced the amounts of PAHs in biochar, the toxic equivalent value increased because Fe induced more accumulation of high-molecular-weight PAHs in the biochar. This study proved that Fe loading suppresses PAH generation during biomass pyrolysis, which can guide the design of composite biochar production.